Hi Tim,
Currently Designer control is for WPF(can be used in Win Forms too), if you want to use this over web then you can deploy as XBAP app(Full Trust) and use it.
http://msdn.microsoft.com/en-us/library/aa969776.aspxI regret we don’t have ASP.Net version of this, but In our near feature we are planning to have a native Silverlight Designer control, let us if you are interested.
Thanks,
Thameem